In the past, laminated foam cushions composed of two or more foams of different hardness have been known and utilized as, for example, in vehicle seats.
Among many methods for preparing such multilayer or multihardness foam cushions, the first one involves foaming different foams of different hardness in separate molds, cutting respective foams into required shapes and dimensions, piling up various cut foams and bonding the same together to form a unitary foam cushion. This method is extremely time and labor consuming in that it requires separate molding and foaming steps, cutting steps and bonding steps; and, moreover, the products of this method cannot be expected to have high quality and be comfortable.
The second method in the prior art involves preparing a first foam of first hardness, placing the same in a mold, pouring the second stock of second hardness onto the first foam and causing the stock to foam thereat to thereby produce a multihardness unitary foam article. The second method is also disadvantageous in that it is time consuming and it cannot define, as intended, a clear and distinct interface or boundary between different layers of different hardness.
The third method is described in U.S. Pat. No. 3,257,149 issued June 21, 1966 which utilizes partitions in the mold to prevent two or more foaming formulations from admixing with each other. This method has widely been employed in the industry and deemed conventional.
Well advanced as compared to the prior methods as mentioned above, the fourth one is a method described in Japanese Laid-Open Publication No. 96195/80 "Multidensity Foam Article and Method of Preparation" (Application No. 123753/79). Briefly, this method is shown in FIG. 10 attached hereto and comprises partially filling a mold a having a lid b with a first formulation c which will yield a firm foam having a high modulus, allowing this formulation to foam to some extent, then pouring onto the foaming first formulation c a second formulation d which will yield a soft foam having a low modulus. The second formulation d which is a liquid and of higher density passes through the foaming first formulation c of lower density to the bottom surface of the mold a. Thus, the foaming first formulation c floats upon the second formulation d. The lid b is closed and both formulations c and d are allowed to foam, rise to fill the entire mold, and cure to thereby produce a composite foam article as shown in FIG. 11 which has a firm foam C and a soft foam D joined together. For use as a seat cushion, the article shown in FIG. 11 will be inverted 180.degree. with the soft foam D on top and firm foam C on the bottom. The inverted cushion will have an upper surface U and a bottom or rear surface R. This type of cushion is purported to have comfortableness and supportiveness owing to its structure comprising the soft, comfortable foam on the top and the firm, supportive foam on the bottom. However, some defects and disadvantage are found in this cushion made according to the fourth method.
(1) While a major portion of the first foaming formulation yielding a firm portion floats upon the liquid second formulation, a thin membrane of the first foaming formulation remains stuck to the bottom surface of the mold. Upon completion of foaming and curing, this sticking membrane produces a thin firm layer C' on the upper surface U as shown in FIG. 11. This firm layer C' is an undesirable product and spoils comfortableness, supporting and fitting feelings intended to be produced by the soft layer D. Uncomfortable vehicle seats do not assure good safety and may lead to traffic accidents. PA0 (2) The second liquid formulation d poured after the first formulation starts to foam penetrates the foaming formulation and sinks toward the bottom. A trace of the second formulation remaining in the first formulation may produce flaws D' on the rear surface R as shown in FIG. 11. This degrades appearance of the foamed article. PA0 (3) Also, the second formulation poured afterwards may break yielding cells in the first formulation which leads to degradation of the firm portion C' of the finished article. PA0 (4) The two portions C and D are not sufficiently strongly bonded together along their boundary because time interval between the first and second pouring is large and the consistencies of the first and second formulations become different from each other. PA0 (5) Waiting intervals are necessary between the first and second pouring, e.g. 30 seconds or more, and this amounts to large time losses in the overall production lines and may require a large number of automated systems.